I personally hate insta kill guards.
RunUO does not have any option either, Instakill only by Default.
I have made new guard AI on my RunUO shard that are very strong, but they also wander the town streets. Some remain standing post in specific areas. While they are pretty much too strong for a one on one combat, they are still killable. They also keep their boundaries in check, and if they chase something, they will stop if they run too far from their boundary. When the guards are spread out even enough throughout town, there will always be another one in the area by time that first one stops chasing.. this was done to prevent anyone from luring the guards in town all away from any given area.
The guards will also loose sight, if you run around a corner and hide, light of sight is also taken into effect to a point.. to kinda create as realistic AI as possible.. rather than them always knowing where you are.. giving you a chance to outrun them, or run and hide behind a small building.. You can actually stealth through town as a murderer, if your good enough, and sneak past them.
I think I have created some pretty damn good guards.. all of this was done because I hate Insta kill guards, and felt something better was in order.
Oh, I have also the opposite.. where if you run into a town thats run by criminals, the guards will actually attack you if your an innocent